Vascular lesions, comprising enlarged or ectatic blood vessels, pigmented lesions, and tattoos have been successfully treated with lasers for many years. In the process called selective photothermolysis, the targeted structure, the lesion tissue or tattoo pigment particles, and the surrounding tissue are collectively irradiated with laser light. The wavelength or color of this laser light, however, is chosen so that its energy is preferentially absorbed by the target. Localized heating of the target resulting from the preferential absorption leads to its destruction.
Most commonly in the context of vascular lesions, such as portwine stains for example, hemoglobin of red blood cells within the ectatic blood vessels serves as the laser light absorber, i.e., the chromophore. These cells absorb the energy of the laser light and transfer this energy to the surrounding vessel as heat. If this occurs quickly and with enough energy, the vessel reaches a temperature to denature the constituents within the boundary of the vessel. The fluence, Joules per square centimeter, to reach the denaturation of a vessel and the contents is calculated to be that necessary to raise the temperature of the targeted volume within the vessel to about 70.degree. C. before a significant portion of the absorbed laser energy can diffuse out of the vessel. The fluence must, however, be limited so that the tissue surrounding the vessel is not also denatured.
As suggested, simply selecting the necessary fluence is not enough. The intensity and pulse duration of the laser light must also be optimized for selectivity by both minimizing diffusion into the surrounding tissue during the pulse while avoiding localized vaporization. Boiling and vaporization lead to mechanical, rather than chemical, damage-which can increase injury and hemorrhage in the tissues that surround the lesion. This constraint suggests that for the fluence necessary to denature the contents of the vessel, the pulse duration should be long and at a low intensity to avoid vaporization. It must also not be too long because of thermal diffusivity. Energy from the laser light pulse must be deposited before heat dissipates into the tissue surrounding the vessel. The situation becomes more complex if the chromophore is the blood cell hemoglobin within the lesion blood vessels, since the vessels are an order of magnitude larger than the blood cells. Radiation must be added at low intensities so as to not vaporize the small cells, yet long enough to heat the blood vessels by thermal diffusion to the point of denaturation and then terminated before tissue surrounding the blood vessels is damaged.
Conventionally, flashlamp-excited dye lasers have been used as the laser light source. These lasers have the high spectral brightness required for selective photothermolysis and can be tuned to colors for which preferential absorption occur. For example, colors in the range of 577 to 585 nm match the alpha absorption band of hemoglobin and thus are absorbed well by the red blood cells in the blood vessels. The absorption of melanin, the principal pigment in the skin, is poor in this range, yielding the necessary selectivity.
The implementation of flashlamp-excited dye lasers presents problems in the pulse length obtainable by this type of laser. Theory dictates that the length of the light pulse should be on the order of the thermal relaxation time of the ectatic vessels. Ectatic vessels of greater than 30 microns in diameter are characteristic of cutaneous vascular lesions. These large vessel have relaxation times of 0.5 msec and require pulse durations of this length. Commercially available flashlamp-excited dye lasers generally have maximum pulse lengths that are shorter than 0.5 msec. As a result, selective photothermolysis treatment of ectatic vessels larger than 30 microns currently relies on higher than optimum irradiance to compensate for the pulse duration limitations. This leads to temporary hyperpigmentation, viz., purpura.
Attempts have been made to increase the pulse durations of flashlamp-excited dye lasers. The Light Amplifier disclosed in U.S. Pat. Nos. 4,829,262 and 5,066,293 was conceived by the present inventor to mitigate laser quenching from thermal effects. The design centered on developing a spatially non-coherent laser. Basically, the optics at each end of the dye cell are designed to return substantially all of the light emanating from the end aperture back through the dye cell and reflect off the dye cell walls. Specific resonating and coherent modes are not favored. The optics mix the rays and thoroughly homogenize the beam. Thus, the effects from thermal distortions induced by the flashlamp are mitigated since resonator modes are not required for lasing action to occur. The invention of this patent does not generate a light that can be concentrated to the degree obtainable by classic laser configurations. But, the large depth of field and tightly focused beams that coherent radiation provides are not necessary for many medical applications. In treating vascular lesions, focussed spots a few millimeters in diameter are adequate. It is often convenient to use fiber optic delivery systems and all that is necessary is to be able to focus the energy from the long pulse dye laser into a fiber approximately one millimeter in diameter.
Newer devices to treat vascular lesions are once again built according to the typical laser paradigm, i.e. lasers that generate spatially coherent light. It turns out that with optimization, these lasers generate pulse lengths that can equal or exceed those achievable by the design producing spatially incoherent radiation described above. Interestingly, dye choice has a large impact on pulse duration. Reduction in dye degradation by improving longevity through dye chemistry has enabled pulse durations approaching 1.0 msec in commercially available devices.